1. Field of the Invention
The present invention relates to an electrophotographic apparatus which employs a novel process in which charging, exposure and development are almost simultaneously carried out with respect to a photosensitive member at one position.
2. Description of the Related Art
There are widely used electrophotographic apparatuses which utilize the Carlson process, in which a charging device, an exposure device, a developing device, etc. are disposed in order around a photosensitive member.
However, there has recently been developed an electrophotographic apparatus which uses a novel process in which exposure is carried out for a photosensitive member from the inside thereof, and in which charging, exposure and development are carried out almost simultaneously at one position, thereby achieving reduction in size of the apparatus.
FIG. 9 shows an electrophotographic apparatus which uses such a novel process. The electrophotographic apparatus has a photosensitive drum 90 which is formed by successively stacking a transparent conductive layer 92 and a photoconductive layer 93 on the outer surface of a transparent cylinder 91.
Further, a developing roller 95 is disposed to face the outer surface of the photosensitive drum 90, and an exposure device 94 for image exposure is disposed inside the photosensitive drum 90 so as to face toward the developing roller 95.
The electrophotographic apparatus uses a developer 96 which is a two-component developer formed by mixing together electrically insulating magnetic toner and a magnetic carrier of low electric resistance. The developing roller 95 has a structure in which a conductive sleeve is driven to rotate around a fixed magnet roller. A developing bias voltage source 97 applies a developing bias voltage of -400 V, for example, to the developing roller 95.
In the above-described apparatus, all the processes for the photosensitive drum 90, i.e., charging, exposure, development and cleaning of residual toner, are carried out in an area (developing nip) where the developer 96 on the developing roller 95 is in contact with the photosensitive drum 90.
First, charging is carried out as follows: The developing bias voltage that is applied to the developing roller 95 is transmitted to the photosensitive drum 90 through the conductive carrier mixed in the developer 96, thereby charging the photoconductive layer 93 provided over the surface of the photosensitive drum 90.
Image exposure that is carried out by the exposure device 94 is effected with respect to the developing nip portion. Thus, the voltage at the surface of the photosensitive drum 90 becomes close to zero (e.g., -20 V) only at the exposed portion, thereby allowing an electrostatic latent image to be formed.
While the exposed portion is passing through the developing nip, the toner is strongly attracted to the exposed portion from the developing roller side by electrostatic force. Thus, the electrostatic latent image is developed by the toner.
While passing through the developing nip, the portion of the photosensitive drum 90 other than the exposed portion is charged to a voltage close to the developing bias voltage applied to the developing roller 95, e.g., about -380 V.
Accordingly, at the non-exposed portion of the photosensitive drum 90, the magnetic force with which the magnet roller of the developing roller 95 attracts the toner, which is magnetic powder, is stronger than the electrostatic force with which the photosensitive drum 90 attracts the toner. Therefore, no toner adheres to the non-exposed portion. As a result, the toner is attached to only the exposed portion of the photosensitive drum 90, and thus a toner image corresponding to the exposure image is formed.
FIG. 10 is a diagram showing the relationship between the voltage at the exposed portion and the voltage at the surrounding non-exposed portion in a typical Carlson process. FIG. 11 is a diagram showing the relationship between the voltage at the exposed portion and the voltage at the surrounding non-exposed portion in the above-described novel process.
In the Carlson process, as shown in FIG. 10, the photosensitive drum charging voltage (i.e., the voltage at the non-exposed portion) V0 and the developing bias voltage Vb applied to the developing roller are set independently of each other. For example, the voltage V0 at the non-exposed portion is set to -600 V, and the developing bias voltage Vb is set to -400 V. The voltage V1 at the exposed portion is, for example, -20 V.
Accordingly, in the Carlson process, the toner in the non-exposed portion is surely attracted to the developing roller by electrostatic force. Therefore, the "smudge" phenomenon that toner undesirably adheres to the non-exposed portion of the photosensitive drum, to which toner must not be attached, is unlikely to occur.
In the novel process, as shown in FIG. 11, the photosensitive drum charging voltage (i.e., the voltage at the non-exposed portion) V0 and the developing bias voltage Vb cannot be set independently of each other. Accordingly, even at the non-exposed portion, electrostatic force acts on the toner so that the toner is attracted to the photosensitive drum 90.
However, since the charging voltage V0 is very close to the developing bias voltage Vb and thus electrostatic force acting on the toner in the non-exposed portion is extremely small, as described above, the toner is pulled away from the non-exposed portion and collected to the developing roller side by magnetic force that has been set so as to be stronger than the electrostatic force.
In the novel process, however, the electric resistance of the carrier gradually increases as printing is continued for the reasons that foreign matter may adhere to the carrier, and that carbon may fall off from the carrier due to stirring, and further that small carrier particles may slip off.
As the electric resistance of the carrier increases, the charging voltage of the photosensitive drum 90 lowers because the photosensitive drum 90 cannot sufficiently be charged through the carrier. Accordingly, in the non-exposed portion, electrostatic force that attracts the toner to the photosensitive drum 90 increases, causing "smudge" to occur. As a result, it becomes necessary to exchange the whole developer for a new one. Thus, the lifetime of the developer is disadvantageously shortened.